One of the known mechanisms for measurement of thickness of slab of material involve usage of low-coherence interferometry based probes or sensors that act as distance measurement-gauge. More specifically, the sensors may be used to obtain a distance between sensor and the slab of material and based thereupon cause determination of thickness. In addition, the sensors are also usable for determining refractive-indices of the materials, and evaluate roughness of the interfaces between different material layers and alike.
FIG. 1 depicts a conventional-arrangement for inspecting the slab of material in the form of a dual-sensor arrangement, wherein the sensors 101-1, 101-2 are positioned one above and one below a slab of material 102. The slab 102 is residing on wafer chuck 103 mounted to mounting rod. Both sensors 101-1, 101-2 are positioned at a fixed-distance ‘D’ there-between. While the distance ‘D’ remains constant during measurement, the light emanating from bottom sensor 101-1 is in optical communication with the slab 102 through a passage in the chuck 103. Upon measuring a distance ‘d1 between the sensor 101-1 and a top of the slab of material and a distance ‘d2” between the bottom of the slab of material 102 and the sensor 101-2, the thickness of the sample t_sample is given by:t_sample=D−d1−d2  (Equation 1)
The distance ‘D’ may in turn be realized as a constant-distance (as a part of factory-settings) by placing a calibration standard of known thickness ‘t_calib’ in the tool of FIG. 1 and finding the value D from Equation 2 which is equivalent to Equation 1:D=t_(calib standard)+d1+d2  (Equation 2)
Such dual-sensor arrangement advantageously eliminates influence of thermal-drift of a length of the optical path from the sensor till the surface of the slab of material, since t_sample is extracted as a difference in Equation 1 and thereby rendered independent of the thermal drift subjected to either side of the slab of material.
The subject matter claimed in this disclosure is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described in this disclosure may be practiced.